In general, heat pumps are devices that impart heat to a high-temperature object by taking up heat from a low-temperature object using a cycle consisting of the steps of evaporation, compression, condensation and expansion. Due to their comparatively high energy utilization efficiency, heat pumps are widely used in heat utilizing apparatuses such as air-conditioners having cooling and heating functions and refrigerators (see, for example, Japanese Unexamined Patent Application, First Publication No. 10-253155).
In a heat pump, heat is absorbed from the surroundings by the latent heat of evaporation during evaporation of a cooling medium. In the case of using in an air-conditioner, heat absorbed during evaporation is supplied from interior air during cooling, and is supplied from the atmosphere during heating. In addition, heat pumps generate heat during condensation of a cooling medium. In the case of using in an air-conditioner, heat generated during condensation is released into the atmosphere during cooling and released into the interior during heating. Examples of cooling media involved in the transfer of heat include fluorocarbon-based compounds as well as ammonia.
The energy utilization efficiency of heat pumps is typically represented with the coefficient of performance (COP), which is the ratio of output heating value to input motive power. Conventionally, high-performance heat pumps had a COP of 2.5 to 4.0. Accompanying the increasing awareness of environmental issues, there is a need to further improve energy efficiency.